Abstract
In this work we report new γ′ strengthened cobalt base superalloys that have low mass densities in the range of ~ 8.3–8.5 gm/cm3, high solvus up to ~ 1128 °C and high specific 0.2% proof strength of ~ 90 MPa.gm-1.cm3 at temperatures up to 770 °C. The basic alloy composition is Co–10Al–5Cr–2Ta that exhibits γ/γ′ microstructure on heat treatment. Electron diffraction and elemental partitioning across γ/γ′ indicate the γ′ has an ordered L12 structure with a stoichiometry Co3(Al,Ta,Cr) coherently embedded in disordered face-centered-cubic (fcc) γ-Co matrix. The γ′ structure is primarily stabilized by Ta addition. The γ′ volume fraction and the solvus temperature of the base alloy was measured to be 14% and 870 °C respectively. These values are significantly enhanced by the addition of 30at.% Ni (35% and 975 °C) and together with 4at.% Ti attaining the values of 63% and 1128 °C, respectively. Atom Probe measurements reveal both Ni and Ti strongly partition to γ′ precipitates enhancing the stability of the γ' while Cr partitions to the γ phase. At 900 °C after 200 h of aging, the 30at.% Ni added alloy without Ti reveals formation of coarsened γ′ at the grain boundaries, directionally coarsened γ′ with wavy γ/γ′ interface and coagulated rod shape γ′ inside the grains. Addition of Ti (2 and 4 at. %), however, suppresses the formation of these features up to 1000 h of aging at 900 °C. The feasibility of tuning the properties of the present alloys by alloying additions makes them attractive for their further development.
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